1. Field of the Invention
The present invention relates to a reproducing amplifier and a magnetic recording/reproducing apparatus employing the reproducing amplifier, and more particularly to a reproducing amplifier for amplifying a signal from a reproducing head disposed near to a recording head and a magnetic recording/reproducing apparatus employing the reproducing amplifier.
2. Description of the Related Art
Recently, the main current of the magnetic recording/reproducing apparatus, such as a hard disk drive (HDD) apparatus using a hard disk as a recording medium, for recording and reproducing information to the recording medium has used an inductive head for recording data on the recording medium being a magnetic material by generating a magnetic field produced by an electric current flowing through a coil as a recording head, and has employed a magneto resistive (MR) head (hereinafter referred to as an “MR head”) having a characteristic such that the resistance value of the head changes according to changes of a magnetic field as a reproducing head, respectively.
FIG. 7 shows a relation of the connection of a head unit and a read/write (R/W) amplifier in a magnetic recording/reproducing apparatus employing an inductive head and an MR head. In FIG. 7, a head unit 103 including an inductive head 101 and an MR head 102 is connected with an R/W amplifier 105 with flexible printed board wiring 104. Signals are transmitted between the head unit 103 and the R/W amplifier 105 through the flexible printed board wiring 104. In the head unit 103, the inductive head 101 and the MR head 102 are disposed in close vicinity to each other.
At the time of data recording in the magnetic recording and reproducing apparatus constructed as above, the R/W amplifier 105 changes the polarity of an electric current flowing in a coil of the inductive head 101 on the basis of write data input from the outside, and thereby the R/W amplifier 105 makes the inductive head 101 produce a magnetic field to record the data on a magnetic disk with the inductive head 101. At the time of reproduction of data, the R/W amplifier 105 receives a signal read by the MR head 102 through the flexible printed board wiring 104, and then the R/W amplifier 105 amplifies the received signal to output an amplified signal between its terminal RDX and its terminal RDY as a differential signal.
FIG. 8 is a circuit diagram showing a circuit example of a reproducing amplifier in the R/W amplifier 105. The reproducing amplifier of the circuit example includes a feedback circuit 202 that supplies a bias current IB flowing in the MR head 102 from a current source 201 connected with a positive power source VCC and controls the electric potential of the middle point of the MR head 102 at the electric potential level of the ground (GND). The reproducing amplifier adopts a current bias voltage sense (CBVS) system in which variation of resistance of the MR head 102 are taken out as voltages.
The MR head 102 is connected between a terminal MRX and a terminal MRY. The feedback circuit 202 is comprised of a transistor Q1, resistors R1-R4, a capacitor C1 and a voltage/current converting circuit gm1. The feedback circuit 202 performs the feedback operation thereof in order that the electric potential at the middle point of the MR head 102 becomes the GND electric potential level. The resistors R2 and R3 are voltage-dividing resistors having resistance values equal to each other.
A signal input from the MR head 102 to the terminal MRX and the terminal MRY is applied to each base of a differential pair of transistors Q2 and Q3 through capacitors C2 and C3, respectively. Each base of the differential pair of transistors Q2 and Q3 is connected with one end of each of resistors R5 and R6, respectively. The other end of each of the resistors R5 and R6 is connected with the negative pole side of a direct-current power supply 15. The positive pole side of the direct-current power supply 15 is connected with the positive power source VCC.
Each of emitters of the differential pair of transistors Q2 and Q3 is commonly connected. A current source 204 is connected between the common connection point and the GND. The differential pair of transistors Q2 and Q3 constitute an amplifier circuit 207 together with a load circuit 205 of the differential pair of transistors Q2 and Q3 and an amplifier 206. Then, a differential signal output from the amplifier circuit 207 is output from the terminal RDX and the terminal RDY through a switching circuit 208 as a signal read by the MR head 102.
In the reproducing amplifier constructed as above, first, a biasing method of the MR head 102 is described. Supposing that an electric current of the current source 201 is designated by a reference character I1 and a collector current of the transistor Q1 is designated by a reference character I2, each electric potential at the terminal MRX and at a terminal + of the voltage/current converting circuit gm1 rises when, for example, the current I1 is larger than the current I2. Because the output current of the voltage/current converting circuit gm1 thereby increases, the base electric potential of the transistor Q1 increases. Consequently, the collector current I2 of the transistor Q1 increases. The operation mentioned above is repeated until the electric potential at the terminal + of the voltage/current converting circuit gm1 becomes the GND electric potential.
If each resistance value of the resistors R2 and R3 is sufficiently larger than the resistance value of the MR head 102, the following relation is satisfied:I1=I2=IBThat is, the feedback circuit 202 performs the feedback operation such that the electric current I1 of the current source 201 and the collector current I2 of the transistor Q1 become equal to each other. Because each resistance value of the resistors R2 and R3 is equal to each other, the electric currents flowing through the resistors R2 and R3 are equal to each other. Consequently, the electric potential at the terminal + of the voltage/current converting circuit gm1 is equal to the electric potential at the middle point of the MR head 102. That is, the electric potential at the middle point of the MR head 102 becomes substantially equal to the GND electrical potential level.
Successively, a circuit operation of the reproducing amplifier is described. First, the resistance value of the MR head 102 varies according to variation of a magnetic field. The reproducing amplifier takes out the variation of the resistance value of the MR head 102 as voltages, and amplifies the voltages to output them. That is, supposing that a variation quantity of the resistance value caused by the magnetic field is designated by a reference character ΔRMR and a gain of the amplifier circuit 207 is designated by a reference character Av, a differential signal having the largeness of IB×ΔRMR×Av is output between the terminals RDX and RDY (or output terminals).
The capacitor C2 and the resistor R5 and the capacitor C3 and the resistor R6 are severally constitute a high pass filter for cutting a direct-current component. Ordinarily, the cut-off frequency fo of each of the high pass filters is set at about 500 kHz (=1/(2π×C2×R5), 1/(2π×C3×R6)). When the R/W amplifier 105 is performing recording, the amplifier 206 and the switching circuit 208 are in their off-states.
As shown in FIG. 7, the inductive head 101 and the MR head 102 are in close vicinity to each other, and each wiring to terminals WX and WY is disposed in parallel to each wiring to the terminals MRX and MRY. Consequently, when recording is performed, potential differences to be generated between the terminals WX and WY varies according to a record signal, and the electric potentials of the terminals MRX and MRY (being the input terminals), which are disposed in close vicinity to the terminals WX and WY, of the reproducing amplifier are affected through the capacity coupling of the terminals WX, WY, MRX and MRY owing to the variation of the potential differences between the terminals WX and WY.
FIG. 9 is a timing chart showing timings of switching between a reproducing (read) mode and a recording (write) mode of the R/W amplifier 105. In FIG. 9, a mode signal to be input into a terminal RXW of the R/W amplifier 105 (see FIG. 7) takes an “H” level for a read mode period and an “L” level for a write mode period. Moreover, the switching circuit 208 takes its on-state for a read mode period and its off-state for a write mode period and for a fixed period T1 after a change from a write mode to a read mode.
In a magnetic recording/reproducing apparatus such as a HDD and the like, it is a primary factor for increasing recording capacity of a recording medium to output a reproduced signal as soon as possible after completion of writing. However, in the magnetic recording/reproducing apparatus, a voltage to be generated between the terminals WX and WY at a time of recording generates a differential voltage on bias voltages of the bases of the differential pair of transistors Q2 and Q3 owing to a cross talk between the flexible print board wirings 104 shown in FIG. 7 and a cross talk between the pins of the terminals WX, WY, MRX and MRY. The time constants of the base electric potentials of the differential pair of transistors Q2 and Q3 are substantially determined to be C2×R5(=C3×R6). The amplifier circuit 207 amplifies the difference between base voltages of the differential pair of transistors Q2 and Q3 as if the difference is a signal when the R/W amplifier 105 enters into a read mode with the difference between the base voltages existing.
Moreover, because a path determining the bias current IB of the MR head 102 is in its operating state even if the R/W amplifier 105 is performing recording, the quantities of cross talks to the terminals MRX and MRY of the reproducing amplifier owing to the recording or the like differ from each other. Consequently, when the electric potential at the terminal + of the voltage/current converting circuit gm1 varies, the base electric potential of the transistor Q1 changes from the electric potential at the time of reproducing. Thereby, the collector current I2 of the transistor Q1 changes when the R/W amplifier 105 shifts to its read mode, and the biasing state of the MR head 102 is changed.
Because the conventional magnetic recording/reproducing apparatus has such problems, it is needed for the apparatus to take the measure such that the apparatus sets a delay time of the fixed period T1 to the switching circuit 208 for outputting a signal after waiting the settlement of the base voltages of the differential pair of transistors Q2 and Q3 after its shifting from a write mode to a read mode, or a measure concerning the connection between the head unit 103 and the R/W amplifier 105. Incidentally, the fixed time T1 is needed to be set at the most suitable value according to the system of a magnetic recording and reproducing apparatus because the fixed time T1 depends on the construction of the head unit 103 and the quantity of a cross talk.